Polarization diversity can be used to provide two parallel communication channels over the same link with orthogonal polarizations, thus doubling the link capacity. Separate and independent signals are transmitted over the two channels thus defined. Despite the orthogonality of the channels, however, some interference between the signals occurs almost inevitably. In order to cancel the effects of this interference, the receiver may comprise a cross-polarization interference canceller (XPIC), which processes and combines the signals from the two receive antennas in order to recover the original, independent channel signals.
A variety of XPIC circuits are known in the art. For example, U.S. Pat. No. 4,914,676, whose disclosure is incorporated herein by reference, describes a cross-polarization interference canceller, which includes a phase difference detecting circuit for detecting a phase difference between main and interference polarization components. In response to the output of the phase difference detecting circuit, a phase shifter shifts a data discrimination clock signal, which is used by an A/D converter in digitizing the interference signal.
As another example, U.S. Pat. No. 5,920,595, describes a receiver that receives two orthogonally-polarized signals. One of the signals is equalized by a digital equalizing unit. The other signal is input to a digital “inter-cross wave compensation” unit, which outputs a compensating signal to be added to the equalized signal. In one embodiment, before this latter signal is input to the inter-cross wave compensation unit, its frequency is corrected by a phase rotating unit. The structure of the receiver is said to allow digital demodulation to be applied to the inter-cross wave compensation, thereby making it possible to form the apparatus as a LSI device and reduce the size and cost of the apparatus.
Other representative XPIC circuits are described, for example, in U.S. Pat. No. 5,710,799, in European Patent Application EP 1365519 A1, and in PCT Patent Application WO 00/77952 A1, whose disclosures are incorporated herein by reference.